12 MASS. EXPKRIMF.XT STATION BULLETIN 303 



at dilutions of 1-SO and 1-40, but slight injury followed the use of gas in two 

 cases out of five. Lack of injury beyond a slight degree in two instances is attrib- 

 uted to the relativa^ly small concentration of copper present as compared to a 

 4-50 strength of copper sulfate in Bordeaux. Stronger solutions were unsafe for 

 cucumber plants. 



Fungtrogen, containing 0.47 per cent active copper and 0.47 per cent nickel 

 and alkaline to litmus, caused no injury of itself or with HCN at dilutions of 

 1-30 and 1-60. The low concentrations, like the low concentrations of copper 

 sulfate, undoubtedly account for its safe use with gas. 



Laboratory-prepared ammoniacal copper carbonate was not to.xic to cucumber 

 plants of itself or with HCN in any strength of copper carbonate ranging from 

 0.08 to 0.24 per cent in 0.8 per cent ammonium hydroxide solution. 



It is evident from these results that more than mere traces of soluble copper 

 must result from the action of gas on copper residues in order to cause injury, and 

 that the copper content of the fungicide is a contributing factor in the reaction. 



Lime 



When sprays of chemical hydratcd lime of 0.2, 0.5 and 1 per cent strengths 

 were used, no injury occurred from the sprays either alone or with gas. 



EFFECT OF HYDROCYANIC ACID GAS ON COPPER SPRAY RESIDUES 



On Cucumber Leaves 



An explanation for injury to sprayed plants from the action of HCN gas on 

 the spray residue was sought in qualitative tests of the residue after fumigation. 

 The dry spray residues from the surface of 10 grams of leaves were carefully col- 

 lected with distilled water, the washings filtered, and the filtrate tested for soluble 

 copper with potassium ferrocyanide. 



The residues of Bordeaux on cucumber leaves acted upon b\' HCN gas yielded 

 positive tests for water soluble copper (Table 1). In three experiments a test 

 was made of the residue of Bordeaux 4-1-50, a neutral mixture, on cucumber 

 leaves. In one of three instances in which the plants were not exposed to gas, no 

 soluble copper was found, and in the other two only a slight trace; while the 

 action of the gas resulted in the presence of much soluble copper and in injury in 

 all three instances. Bordeaux mixtures containing higher amounts of lime, e.g., 

 4-2-50, 4-3-50, 4-4-50, 4-6-50, 4-8-50, and 4-12-50, when exposed to gas yielded 

 soluble copper, and cucumber foliage was injured in every instance. The degree 

 of injury and the amount of soluble copper in the residue increased as the ratio of 

 lime to copper sulfate was increased. The residues of neutral and alkaline Bor- 

 deaux 4-X-50 mixtures, prepared with milk of lime or filtered lime water and either 

 washed or not, consistently yielded soluble copper after the action of gas, and 

 its presence was associated with foliage injury. Similarly the residue of a neutral 

 4-X-50 Burgundy mixture yielded but a trace of soluble copper, while the action 

 of gas resulted In much of It; and the residue of neutral caustic soda Bordeaux 

 4-X-50 showed only a trace of soluble copper in one Instance and none in the other, 

 while after the action of gas strong tests were obtained In both cases (Table 2). 



The results with basic copper sulfates (Table 3) are particularly interesting 

 because the lack of Injury from these materials was definitely correlated with the 

 compatibility of their residues on leaves with gas. In three of four tests only a 

 trace of copper was detected, and In the fourth test none. However, when chemical 

 hydrated lime was added to basic copper sulfate and the sprayed plants exposed 

 to gas, good tests for soluble copper were obtained in each of seven experiments, 



